Of course, those obvious things that changed Europe and world history (100 Years' War, Black Death, Crusades etc), but also on a more personal level. Life back then was pretty hard, and even the tale of the most incignificant peasant from the time would make a good book with a lot of dramatic actions within it. Overall, the medieval culture is very fascinating with its broad acceptance of violence and murder while the church preached about being a good christian (and making a lot of money in the process).

Most interesting sinlge person is Karl Knutsson Bonde I would say. King of Sweden no less than three times, and king of Norway for a short period.